1. Field of the Invention
The present invention relates generally to measuring optical properties of a surface, and specifically to measuring total emittance from a surface.
2. Description of the Prior Art
Measuring surface optical properties (e.g., reflectance, transmittance and emittance) is vitally important in the aerospace industry. In order to control the surface temperatures of satellites, spacecraft and high-speed aircraft, airframe designers must be able to measure the heat absorbing and emittance properties of the surfaces they employ in their designs. Also, it is important to monitor the surface optical properties of reusable spacecraft (e.g., the space shuttle) to ensure continued safe operation. Such measurements are also important in solar energy applications and surface chemical analyses.
Currently, devices used to measure the surface reflectance of reusable space craft are not portable. To measure such reflectance, a technician must remove a sample from the spacecraft, patch the resulting hole, and mount the sample in a measurement device in a laboratory or more commonly, use a witness sample technique. Because of the complexity involved, such measurements are limited to only a few samples.
Several devices have been disclosed that measure some of the reflectance properties of surfaces. These include U.S. Pat. No. 4,360,275 issued to Louderback, which discloses a device for measuring total diffuse optical scattering from the surface of a sample, including a surface having extremely low scatter. A light beam from a laser is directed through an entrance hole in an ellipsoidal reflector onto the sample. A sample mounting means is used to position the sample so that the specularly reflected portion of the light beam is directed out of an exit hole located in the reflector diametrically opposite the entrance hole.
Prior to the measurement of scattering, the Louderback device must first be calibrated by measuring its output for a known amount of light scattering. This is accomplished by inserting in place of the sample a scattering sample having a known amount of scattering, such as a disk covered with barium sulfate (BaSO.sub.4) having a 99% diffuse scattering. In addition, an optical filter having a known, low transmittance is inserted within the beam in between the output of the laser source and the entrance hole. The optical filter avoids exposing the sensitive photomultiplier tube to a high level of light that would saturate or even damage it. With the beam incident on the calibration sample, the voltage of the photomultiplier tube is adjusted so that the device reads the known transmittance of the optical filter directly.
U.S. Pat. No. 3,504,983 issued to Richmond et al. discloses a reflectometer in which a luminous flux is directed through an aperture in an ellipsoidal mirror to a specimen supported by a support means at the first focus of the ellipsoid. Light reflected off of the specimen is reflected off of the ellipsoidal mirror to a collecting sphere at the second focus of the ellipsoid. The collecting sphere directs the light to a detector.
U.S. Pat. No. 3,010,358 issued to Seigler discloses a radiation comparison system having off-axis ellipsoidal mirrors with a sample supported at one focus.
U.S. Pat. No. 4,177,383 issued to Knight discloses a device for treating a sheet material with radiation. An elliptical reflector directs light from an ultra-violet light source onto a surface being treated by the light.
U.S. Pat. No. 4,578,584 issued to Baumann et al. discloses a non-contact thermal imaging system in which an energy source is scanned across a sample supported by a pedestal. Suitable energy sources include electron beams, ion beams and laser beams. The scanned beam discontinuously heats discrete points of the surface of the sample. A thermal wave is generated on the surface of the sample and means are provided to image the two-dimensional area of the sample. A hemi-ellipsoidal reflector acts as a collector of infrared radiation with the sample placed at one focus and the detector placed at the opposite focus. The scanned beam passes through an aperture in the reflector.
U.S. Pat. No. 4,988,205 issued to Snail discloses a reflectometer in which the sample is placed at one focus of a hemi-ellipsoidal primary mirror. The sample is illuminated by a beam reflected into the cavity of the primary mirror. Light reflected off of the sample is directed, by the primary mirror, into a secondary mirror having an entrance aperture placed at the second focus of the primary mirror. The secondary mirror directs all entering light to a detector. In a second embodiment, the primary mirror is a dual paraboloid comprising two opposing mirror surfaces with the sample being placed at the focus of one paraboloid and the secondary mirror being placed at the focus of the second paraboloid.
U.S. Pat. No. 5,127,729 issued to Oetliker et al. discloses a light collector used in photometry having an elliptical reflector. An external light source illuminates a sample placed at one focus of the ellipse. A detector may be placed at the other focus. The sample is placed in a glass tube which is inserted through a hole in the elliptical reflector.
U.S. Pat. No. 5,216,479 issued to Dotan et al. discloses an optical inspection system used for inspecting the laminae of a printed circuit board (PCB). A laser beam is directed to the surface of the PCB. Specular and diffuse radiation is directed by a partial ellipsoidal reflector to filters and mirrors that reflect the radiation to detectors.
The DB100 Infrared Reflectometer, produced by the Gier Dunkle Instruments division of Dynatech Instruments, Inc., has an inspection head containing two rotating semi-cylindrical cavities. One of the cavities is heated by an electrical heater and the other stabilizes at approximately room temperature. Thus, the two cavities are maintained at different temperatures. As the cavities rotate, the sample is alternately irradiated. A vacuum thermocouple views the sample through an optical system that focuses through slits in the ends of the cavities. The detector receives energy emitted by the sample and energy reflected by the sample.
Of the above devices, only the DB100 Infrared Reflectometer is employed in a portable device for measuring the total reflectance (both diffuse and specular) properties of an arbitrary surface. The DB100 measures specular and partial (less than 2.pi. steradian) diffuse reflectance and is limited in wavelength range by the deflector window. None of the above devices provide an absolute measurement of reflectance.
Thus, there exists a need for a device that measures both the total diffuse and specular reflectance properties of a surface.
There also exists a need for a device that measures the total reflectance properties of surfaces having arbitrary topography.
There also exists a need for a device that is easily calibrated to provide an absolute measurement of reflectance.
There also exists a need for a total reflectance measuring device that is portable.
There also exists a need for a coating for transducing light, especially infrared and visible light, into infrared light, which resists thermal shock and will not exhibit spalling or delamination when subjected to high temperatures.